So when you get pulled over in your car, there are limits, of course, to what a police officer can search. Sometimes the officer calls in extra help - right? - drug-sniffing dogs. Well, it turns out the handlers who work with those dogs can have a hard time knowing when they actually smell something illegal. And to figure it out, the handlers are making training methods more scientific. Let's hear more about this from NPR's Martin Kaste.

MARTIN KASTE, BYLINE: Andy Falco is a former police dog handler, now a trainer and an expert defense witness in criminal cases. And all that experience has made him a little cynical about K-9s.

ANDY FALCO: It's such an easy tool to abuse.

KASTE: He says the dogs themselves are great. The problem is the humans who are sometimes tempted to use dogs as a legal shortcut to justify a search. Some cops even joke about dogs being probable cause on four legs.

FALCO: It's easy. All I've got to do is walk my dog around. You know, he blinked his right eye twice as we went by the door, so now we can go inside.

KASTE: He says he sees this attitude among police who specialize in pulling over cars in search of drug money, money that police departments can often seize and keep. But does Falco really believe that those K-9 handlers are being consciously deceptive?

FALCO: I think in the beginning, I think it's just subconscious. But at some point, you know, how hard do you want to work, right?

KASTE: This is a big question in the world of sniffer dogs. Yes, some cops might be purposefully prompting their dogs to alert. But this kind of cueing, as it's called, can also be unconscious. Cindy Otto studies working dogs at the University of Pennsylvania. She says even officers who are acting in good faith might be influencing their dogs without even realizing it.

CINDY OTTO: And that's, I think, what's the most magical part about dogs is because they're so attuned to humans - it's also the dangerous side - because they're so attuned to humans, they can sometimes read cues that we don't realize we're giving.

KASTE: There was a study about this from the University of California, Davis in 2011. It was just one study, but it made headlines. And it's now often cited by defense lawyers. And what it was was this - on the surface, a straightforward test of the detection abilities of 14 trained, certified K-9 teams. But in reality, there was nothing for the dogs to find, and the study was actually looking at the dogs' handlers.

OTTO: They were told there was something to be found. And when they were told that, they had that bias, and they created the situation where there was something to be found.

KASTE: Their dogs alerted to nothing, apparently because their handlers believed that there was something there. The study caused an angry backlash from handlers and K-9 associations. But now that some time has gone by, you can see that it's had an effect. For example, here in a cavernous warehouse in Pasco, Wash., where a K-9 is sniffing and scrabbling around the outside of a parked truck.

(SOUNDBITE OF DOG PANTING)

UNIDENTIFIED POLICE OFFICER: Up, up.

FRED HELFERS: Get them off the leash.

UNIDENTIFIED POLICE OFFICER: Good girl. Up, up. Good girl.

KASTE: This is a certification test run by the Pacific Northwest Police Detection Dog Association. Groups like this periodically test police K-9 teams to make sure they're reliable.

HELFERS: The center part of that scene - a lot of odor there, a lot of odor.

KASTE: But this particular K-9 association is doing things differently than most. It's trying to eliminate any chance of bias, conscious or unconscious, in part by making the tests completely randomized.

(SOUNDBITE OF DOG PANTING)

HELFERS: OK. Roll the dice. You're going to two. You're going to A.

UNIDENTIFIED POLICE OFFICER: OK.

HELFERS: Go in, search it, and then come on out. Shut the door.

KASTE: A roll of the die determines which room or cars the dog will search. And there are no patterns to where things are hidden. Also, no one in the room can know the location of the drugs, not even the person who's administering the test. This system was designed by Fred Helfers, a veteran police dog handler and trainer. His goal here is what scientists call double-blind testing.

HELFERS: The thing about a double blind is no outside influence - just him and his dog - just like it is in real life.

KASTE: And here's the key. Sometimes, there's nothing hidden. Depending on the dice, there have been occasions when there have been no drugs hidden for several tests in a row. Helfers recalls a recent streak of blanks.

HELFERS: There were some new teams that failed that sequence because they didn't trust their dog.

KASTE: He says those handlers couldn't get past their expectation that drugs should be there.

HELFERS: You know, I think they - what we call - overworked the car. Instead of going around once or twice and trusting their dog and watching their dog work, maybe they seen something that wasn't there.

KASTE: So they had false positives, the ones who failed that?

HELFERS: They'll false alert, yeah.

KASTE: It's a disturbing thought for anyone who's ever been pulled over and sees that K-9 team coming. But K-9 handlers don't like being told that they might be unconsciously cueing their dogs. Waiting outside the warehouse for the test, one officer said he didn't believe it was even possible. There's also the fact that police work is not the same as science. Officers are trained to act on their suspicions. Gunner Fulmer, a K-9 cop from Walla Walla, describes how things work in real life.

GUNNER FULMER: The dogs are mainly used to confirm what we already suspect. You know, we - yeah, we get our dogs out. And when the dogs come out, about 99 percent of the time, we get an alert. And it's because we already know what's in the car. We just need that confirmation to help us out with that.

KASTE: That's hardly double blind. But it may be too much to expect scientific neutrality during roadside stops. Fred Helfers hopes his testing will at least make K-9 teams prove that they're able to search in a more impartial fashion. As he keeps saying, the handlers need to learn to trust their dogs.